Typically, licensed wireless systems such as Global System for Mobile communications (GSM), Universal Mobile Telecommunications System (UMTS), and Code Division Multiple Access (CDMA) and unlicensed wireless systems such as Wireless Fidelity (Wi-Fi) and Bluetooth are deployed in overlapping coverage areas individually. With its relatively high data rate and low maintenance cost, the popularity of unlicensed wireless systems has increased dramatically over recent years. However, the unlicensed wireless systems are limited in small service area and low mobility. In the meantime, the licensed wireless systems are advantageous in broad service areas and high mobility, in spite of low data rate and high usage cost.
Recently, with the advent of Unlicensed Mobile Access (UMA) technologies converging capabilities of licensed and unlicensed wireless technologies, UMA-enabled dual-mode handsets have been developed.
In UMA technology, a UMA Network Controller (UNC) enables access to voice, data, and other mobile services of a cellular communication system via an Access Point (AP) of an unlicensed wireless system, i.e. Wireless Local Area Network (WLAN), without modification of cellular system equipments such as Mobile Switching Center (MSC), Serving General Packet Radio Service (GPRS) Support Node (SGSN), Gateway GPRS Support Node (GGSN).
Using the UMA-enabled dual-mode handset, a subscriber can roam between cellular networks and unlicensed wireless access networks such as WLAN without breakage of ongoing voice and data services, whereby the UMA technology distributes traffic load of the cellular networks and thus improves service quality.
In the meantime, the unlicensed wireless access systems are superior to the licensed wireless access systems in data rate but limited in small service coverage and small number of users to serve simultaneously. Accordingly, it is practical to perform handover between the licensed and unlicensed wireless access networks when the UMA-enabled dual-mode handset comes within or out of the service area of the unlicensed band system.
For these reasons, there has been a need for an efficient handover technique between the licensed and unlicensed wireless access networks.
Typically, the handover from an unlicensed wireless access network to a licensed wireless access network is determined when any of conditions specified in the communication standard, e.g., when received signal strength of unlicensed wireless access network becomes less than a predetermined threshold. When a mobile station decides to trigger the handover, it sends a URR_HANDOVER_REQUIRED message to the unlicensed wireless access network.
In order to initiate the handover normally, it is required to guarantee signaling between the mobile station and the unlicensed wireless access network from sending the handover request message to receiving the handover command (URR_HANDOVER_COMMAND) message.
Accordingly, in a case where there is excessive signal loss or delay during the signaling session between the mobile station and the unlicensed wireless access network, the active call connection must be released.